In a known manner, femoral prosthesis include a femoral rod for implanting in the medullary cavity. The rod ends in a truncated end portion and is covered by fitting a femoral head having the general shape of a sphere. The ball is provided with a blind bore and has a complementary shape to that of the end portion of the femoral rod. These balls, which are most often made of ceramic material, for example ZrO2, Al2O3, Si3N4, etc., are subjected during use to high mechanical stresses and must consequently answer elevated reliability criteria.
Despite the great care taken during the manufacture of such heads, they inevitably include microscopic defects. These heads consequently have to be tested at the end of manufacture in order to eliminate those with defects exceeding a critical size. In order to do this, several tests have been envisaged.
A first test, called the “Proof test” consists in applying a regular force over the inner surface of the truncated bore of the ball, the force applied being considerably less than the maximum force causing the ball to break. By way of example, this test imposed by the American Food and Drug Administration recommends that the femoral heads to be implanted pass this test and, in particular, resist a minimum force of 20 kN.
This test is generally implemented in an apparatus of the type illustrated schematically in FIG. 1. This apparatus includes a truncated shaft 1 projecting from a base 2. Shaft 1 includes flow channels 4 for a pressurized fluid, opening out onto its truncated surface. The shaft is covered by a sleeve 6 made of plastic material. Sleeve 6 includes in its inner part an annular groove 8, which, with the truncated surface of the shaft defines a pressure chamber in which the pressurized fluid flows. At the location of groove 8, the wall of sleeve 6 is thinned and forms a deformable membrane that can abut against the inner wall of the bore. The femoral head test is carried out in the following manner. The head is fitted onto the shaft and is held in this position by a counter-support. A pressurized fluid is then introduced into the flow channels arranged in the shaft to generate the desired pressure on the lateral wall of the bore. The pressure is transmitted via the deformable membrane of the sleeve. By way of indication, the pressure necessary for implementing the Proof test on this type of apparatus are of the order of 650 bars.
A second test implemented by the manufacturers of femoral heads consists in taking samples from the manufacturing batches and subjecting them to a destructive test aimed at determining the mean breakage resistance per batch, for example as a function of the shape of the truncated housing, the quality of the ceramic material, the roughness of the surface and the micro-cracks caused by the manufacturing process.
The test apparatus described hereinbefore does not allow sufficiently high pressures to be used to determine the maximum pressure at which the ball breaks, since the sleeve made of plastic material does not resist such pressure, which can exceed 5000 bars. It is thus necessary to use another apparatus to carry out these tests. The apparatus used for this includes a shaft provided with a truncated end onto which the femoral head is fitted. The shaft is mobile in translation along its longitudinal axis and can be pushed into the head to apply a force by wedge effect on the lateral wall of the bore while it is held by a counter-support, until the head breaks. The breaking force is recorded and thus provides statistical data allowing the heads of a batch to be characterized and the end quality of the heads to be checked.
Unfortunately, this apparatus cannot be used to implement the Proof test. Indeed, when this test is implemented, metal particles from the friction of the shaft on the inner wall of the bore of the heads are deposited on the wall and make the heads unusable for the desired medical application.
Moreover, the conical part of the shaft is deformed during the test and has to be regularly replaced, typically every two or three tests. Another drawback of this apparatus arises from the very nature of the test, in that the wedge effect used does not allow uniform pressure to be applied over the entire periphery of the inner wall of the bore and inevitably leads to concentrations of stress in certain areas of the head because of surface defaults or other imperfections. These concentrations of stress increase the dispersion of the break load values and thus prejudice the statistical results obtained.
This situation means that the ceramic femoral head manufacturers have to have a test apparatus of each of the aforementioned types, which is impractical, expensive and requires significant handling of parts.
There thus exists in this field an unsatisfied demand for a device for implementing both the Proof test and the break test.
It is thus a main object of the invention to overcome the drawbacks of the aforementioned prior art, by providing a test device having a simple and economical construction and allowing both of the aforementioned tests to be implemented.